1. Field of the Invention
The present invention relates to walls that are used for manufacturing radomes, in particular radomes for aircraft, and radomes using such walls.
2. Description of the Prior Art
Many radar antennas are of the rotary type and undergo disturbances from the outside environment, in particular wind, rain, temperature, etc. In certain applications, these atmospheric disturbances have adverse consequences on the technical performance of radars; for example, the wind can substantially modify the angular velocity of the antenna, which may lead to erratic angular measurements that are incompatible with required accuracy. In the same way, rain water flowing on the surface of the antenna may modify its radio-frequency characteristics to a degree such that the performance of the radar is affected. Thus, in such applications, it is usual to protect the antenna by a dome that is transparent to the radio waves transmitted and received by the radar associated with the antenna.
This protection of the radar antenna of the rotary type by a dome called a "radome" is absolutely necessary for the radars carried by vehicles that moves at high speeds. This is the case of aircraft radars, that are often disposed at the front end of the aircraft in hollow volumes in the form of cones, rectilinear or ogival. These radomes must be designed to resist the very significant mechanical and thermal stresses while retaining their radio-frequency properties. These radio-frequency properties or characteristics are essentially the following:
a good radio-frequency transparency so as to limit the loss of energy of the transmitted and received waves during their passage through the wall of the radome; PA1 a low distortion of the antenna beam, which distortion must be reproducible from one radome to another, and PA1 a low reflection level of the incident wave.
It is easy to compensate for the distortion of the beam, that is mainly characterized by a deflection of the axis of the beam, by offsetting the axis at the radar itself provided, of course, that this deflection is the same from one radome to another. For a radome wall with a given structure, the reproducibility is related to the quality of manufacturing of the wall and not to the composition of its structure.
On the other hand, for both other essential characteristics, namely radio-frequency transparency and reflection of the incident wave, the structure of the wall is very significant.
For aircraft radomes, the structures used are often monolithic. They consist of a layer of dielectric material. The thickness of this layer is, for a good operation, close to a whole multiple of half the wavelength in the dielectric. Such structures have the major disadvantage that the corresponding radomes have a bandwidth of about 3 to 5%, whereas modern radars require a bandwidth wider than 10%.
It is well known that a structure made up of several layers of dielectric material allows an increase in the operating bandwidth of the wall. For example, a structure made up of five dielectric layers will comprise a center layer or core, two intermediate layers having a spacer dielectric role and two end layers or "skins", one inside the radome and the other outside. The index of refraction of the central layer and of the skins is greater than that of the intermediate layers, while the thickness of the central and intermediate layers is greater than that of the end layers. Such structures with five layers are difficult to manufacture in a precise and reproducible manner over the whole surface of the radome due, for one, to the number of layers and, for another, to the precision with which the intermediate layers of foam or honeycomb must be machined, formed to shape and assembled.